Steel tubing is widely used in the petroleum industry for production of oil and gas from subsurface well formations that are typically located many hundreds or thousands of feet below the surface of the earth. The tubing must be supported at its upper extremity, and therefore it is necessary that each section of the tubing string be capable of supporting the entire weight of the string of tubing connected below it. The tension stresses that are placed on the tubing can cause either partial or total rupture of the tubing in the event the tubing was either manufactured with a fissure or flaw in the wall section thereof or if the tubing might have developed a stress crack while in use. For example, the tubing manufacturing process might have caused a section of tubing to be manufactured with a small fissure in its wall structure. When this tubing becomes highly stressed as it supports the weight of the tubing string below it, the small imperfection might open up to the point that leakage occurs. In this event, it is necessary to pull the tubing string and replace the defective tubing section, and logically this is a very expensive procedure that should be avoided if at all possible. As another example, steel tubing that is supported within a well where the production fluid being produced has a high concentration of hydrogen sulfide can cause hydrogen sulfide embrittlement of the tubing to occur, developing minute cracks that in time will begin to leak or cause separation of the tubing. If tubing has been utilized in a well having a high concentration of hydrogen sulfide, it may pass pressure and stress tests. However, when inspected by magnetic detection, otherwise undetectable fissures or subsurface flaws may be detected that will cause the tubing to be rejected for further use.
Drill stem and other pipe may be tested in the same manner to indicate any longitudinal or transverse imperfections in the material from which it is composed. Detection of otherwise undetectable flaws in the structure of drill stem or other pipe can, of course, prevent costly interruptions in drilling or production that render magnetic detection of such flaws extremely advantageous.
The theory of a "longitudinal defect" in metal tubing or pipe is that it creates in effect a single metal bar that is bent around the axis of the pipe. When the pipe is subjected to a magnetic field of constant strength and polarity, the "bar" becomes magnetized, whereby each side of the defect is at a different polarity. It is well known in the art to utilize this concept by passing a coil over a defect in a pipe at a constant speed, thereby cutting the magnetic lines of force in the coil to induce an electromotive force (EMF) into the coil proportional to the magnetic field in the defect. This EMF is of course detected and measured to detect the defect, per se, in non-destructive testing apparatus that is presently being utilized in the industry.
One of the problems with the magnetic detection technique in the prior art is that all parameters must be held constant from point to point (or else known), or else the EMF cannot provide a reliable indication of the size of the defect. For example, two consecutive points may have different magnetisms, whereby the respective EMF thereof will be different -- even when the two defects are of the same size and characteristic. Moreover, magnetic testing utilizing the single "bar" technique is not typically of such sensitivity that very small surface or subsurface fissures can be detected with a high degree of accuracy. It is desirable, of course, to provide non-destructive testing equipment having the capability of detecting metal flaws of any size or characteristic so as to eliminate the possibility of placing any tubing in service that might rupture under pressure or mechanical stress or develop leakage.
Many techniques have been proposed for overcoming this disadvantage in the prior art. For example, some devices provide for rotating either the pipe within the coil or, in the alternative, rotating the coil about the pipe in order to produce a more uniform and therefor constant magnetizing of the pipe. Alternatively, additional magnets producing lines of flux that are oriented at 90.degree. to the direction of the coil are often used to greatly intensify magnetization, whereby the relatively small natural or residual magnetism in the pipe is overridden and swamped. This, in turn, has greatly enlarged and complicated the non-destructive testing equipment.
Wall thickness testing for pipe and tubing is generally accomplished in accordance with the eddy current concept with a pair of spaced coils about the workpiece being energized to induce a magnetic field into the workpiece, and with an intermediate coil between them to pick up any changes in the magnetic field. The fact that the magnetic fields are not induced into the workpiece at the point of a defect but rather remote to it causes this type of testing to be fairly inaccurate.
It is therefore a primary feature of the present invention to provide novel non-destructive testing equipment utilizing the magnetization concept but overcoming the problems that are generally associated with the "magnet bar" concept that is presently employed for non-destructive testing of metal pipe and tubing.
It is also a feature of the present invention to provide novel, non-destructive testing apparatus wherein a small amount of flux is located at the precise location of any defect and is generally oriented in a coinciding manner relative to the defect in order that the defect may be more readily observed.
An even further feature of the present invention contemplates the provision of novel non-destructive testing apparatus for elongated metal objects such as tubing and pipe wherein a magnetic field is generated in the metal object that is extremely small relative to the object as a whole but is very large in relation to the amount of residual magnetism at the defect, per se, thereby causing even a minute defect to produce a rather substantial change in the electronic detection signal, thereby giving a more strong indication of the presence of a defect than is otherwise obtainable.
It is also an important feature of the present invention to provide novel, non-destructive testing apparatus utilizing the magnet and coil detection concept wherein substantially greater penetration of the magnetic field is achieved relative to the metal object being inspected, thus enhancing accuracy of the testing process and providing a more positive indication of the defect than is otherwise typically obtainable.
It is an even further feature of the present invention to provide novel, non-destructive testing apparatus that promotes a general reduction in the overall size of the magnetic testing equipment without in any way detracting from the quality of flaw detection that is available through use of such equipment.
Another important feature of the present invention contemplates the provision of novel testing apparatus for inducing a magnetic field into the workpiece being inspected and moving the magnetic field along the length of the workpiece and for detecting dimensional changes in the thickness of the workpiece by monitoring electrical signals relating to the magnetic field.
Other and further features and advantages of the invention will become obvious to one skilled in the art upon an understanding of the illustrative embodiment about to be described, and various advantages not referred to herein will occur to one skilled in the art upon employment of the invention in practice.